SQL server constrains

Hi, I have two problems that I can't fix, would you like for soe help
I have a table with four fields, two of them are related so only in one of them have to be written value, another have to be NULL. I mean this, if both fields are field1 and field2 the table should look like this:

Is there a way to make constrains or index, I'm not sure, in definition of the table so the field should be written like the example

If it is not possible
The second question is, because of this I'm using MS ACCESS to make records into the table, how in the form
to check before record if the fields are filled incorrect to alert the user that have to edit inserted information.